iNAV erratic behaviour in GPS based modes

[techatron66]

Hello everyone, I have built a 11in drone with 7in props, a speedybee f405 gen4 and emax motors with a Neo m8n gps module.
The drone is airborne with a little bit of drift, but sometimes displays erratic behaviour like when landed and is disarmed the motor keep spinning for some time, this was done with a fresh set of props and 0 vibration.
https://github.com/user-attachments/assets/6302d97d-57a3-4241-88d0-2f571d6944f3
INAV_9.0.0_cli_20260226_104510.txt
I have attached the inav configuration file below.
While testing out the gps based modes, the altitude hold worked fine, but when the position hold was tested the drone flipped 180 in the and just plummeted to the ground followed by the same disarming issue, this time around the drone was fitted with a couple of damaged propellers that induced vibrations into system.
inav mission planner shows 3d gps lock along with the location on the map.
Is there any way to safely test the gps modes without damaging the drone

[sensei-hacker]

Can you please either attach your blackbox log or send it top me on Discord, if you have one.
If you want to, you can remove or alter the line near the top that gives he home location.

[techatron66]

@sensei-hacker thanks for ur instant response, unfortunately I do not have a blackbox log, I will set it up, get the drone running for some time and attach it.

[techatron66]

Got the logs extracted from the drone. Haven't used any gps modes. Hit failsafe in one of the runs
LOG00007.TXT
LOG00008.TXT
LOG00006.TXT
LOG00003.TXT
LOG00004.TXT
LOG00005.TXT
LOG00001.TXT
LOG00002.TXT

WhatsApp.Video.2026-03-17.at.11.30.32.mp4

WhatsApp.Video.2026-03-17.at.11.30.31.mp4

[remete618]

The 180-degree flip on entering position hold is almost certainly a magnetometer, compass calibration or interference issue

i guess you can check by when you switch from altitude hold (uses only baro + accelerometer) to position hold (adds gps + compass for heading reference), iNAV suddenly trusts the magnetometer for heading. If the compass heading is 180 degrees off from the gyro-estimated heading, the FC commands a full inversion trying to "correct" and that will do an instant flip.

Some common causes on a Neo M8N with SpeedyBee F405: Compass orientation wrong in iNAV so you should check align_mag in CLI.

The Neo M8N GPS modules have varying compass orientations depending on manufacturer. Try CW270FLIP or other rotations

Compass calibration - recalibrate outdoors, away from metal/concrete. Rotate on all 3 axes - I assume you've tryied this already though but worth mentioning.

Also damaged props causing vibrations, you mentioned this yourself. Vibrations corrupt accelerometer readings, which corrupts the attitude estimate. iNAV's position controller assumes attitude is correct, bad attitude + position correction = flip

As for the the motor-spin-after-disarm issue, is likely motor_stop not being enabled, or the anti-vibration motor filtering keeping a residual output. Check set motor_stop = ON in CLI.

[sensei-hacker]

The 90° turn in the empirical method of confirming compass alignment will tell you if the compass alignment is set upside-down or backwards.

https://github.com/iNavFlight/inav/wiki/GPS-and-Compass-setup#empirical-method-of-determining-compass-alignment